1. Field of the Invention
The present invention relates to semiconductor photonic devices and a method for forming a ZnO film. More particularly, the invention relates to semiconductor photonic devices using Group III-V compounds such as GaN, InGaN, GaAlN and InGaAlN. The invention also relates to a method for forming a ZnO film deposited on a substrate such as an Si substrate or a glass substrate.
2. Description of the Related Art
As materials for semiconductor photonic devices such as light emitting diodes (LEDs) and laser diodes (LDs) which emit blue light or ultraviolet light, III-V semiconductor compounds represented by the general formula In.sub.x Ga.sub.y Al.sub.z N (where x+y+z=1, 0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1 and 0.ltoreq.z.ltoreq.1) are known. The semiconductor compounds have high light emission efficiency because they are of the direct gap type, and emission wavelengths can be controlled by the In content, and thus these semiconductor compounds have been receiving attention as materials for light emitting devices.
Since it is difficult to form a large single crystal of the In.sub.x Ga.sub.y Al.sub.z N, a so-called "hetero-epitaxial growth method" is used in which a crystal film is grown on a substrate of a different material, and generally it is grown on a c-plane sapphire substrate. However, c-plane sapphire substrates are expensive, and moreover, because of large lattice mismatching, many crystal defects at defect densities of 10.sup.8 /cm.sup.2 to 10.sup.11 /cm.sup.2 occur in the grown crystals, and thus it is not possible to obtain quality crystal films having excellent crystallinity, which is a problem.
In order to reduce lattice mismatching when In.sub.x Ga.sub.y Al.sub.z N is grown on a c-plane sapphire substrate and to obtain crystals having few defects, a method has been developed in which a polycrystalline or amorphous AlN buffer layer or a low pressure growth GaN buffer layer is provided on the c-plane sapphire substrate. In accordance with this method, since lattice mismatching between the c-plane sapphire substrate and the buffer layer is reduced and at the same time lattice mismatching between the buffer layer and In.sub.x Ga.sub.y Al.sub.z N is reduced, a crystal film having few defects can be obtained. However, in addition to the expensive c-plane sapphire substrate in this method, the structure becomes complex, resulting in a further increase in cost.
Additionally, an SiC substrate has been investigated, and there is small lattice mismatching with the SiC substrate. However, SiC substrates are much more expensive in comparison with c-plane sapphire substrates (approximately 10 times as costly as c-plane substrates), which is disadvantageous.
Accordingly, it has been conventionally desired that semiconductor photonic devices be fabricated using inexpensive Si substrates or glass substrates. For that purpose, an In.sub.x Ga.sub.y Al.sub.z N-based light emitting device may be constructed by depositing a ZnO film (buffer layer) having a hexagonal system oriented in the c-axis direction on an Si substrate or glass substrate and by forming a semiconductor containing GaN thereon.
When a ZnO buffer layer is provided on an Si substrate, the substrate cost can be limited to approximately one-tenth of that of a c-plane sapphire substrate, enabling low cost. Additionally, while a c-plane sapphire substrate is an insulating material, an Si substrate can be made conductive and a p-type electrode and an n-type electrode can be provided on the upper side and the lower side of the light emitting device, thus simplifying the structure of the device.
In the past, it was not possible, simply by forming a ZnO film oriented in the c-axis direction on an Si substrate, to grow a thin film containing GaN having good crystallinity thereon. Since the c-axis orientation of the ZnO film sensitively affects the crystallinity of the thin film containing GaN formed thereon, in order to form a thin film containing satisfactory GaN thereon, a ZnO film having the best possible c-axis orientation must be obtained.